/*! \page TBDM_BASISDoc two-body density matrix on a basis

Keyword: TBDM_BASIS

\section description Description

This  object evaluates the one and two-body reduced density matrix on a basis. 
The basis can be any set of functions that can be expressed in a \ref MO_matrixDoc object. 
Usually this will be a set of orbitals from a mean-field calculation.
The ORBITALS section can often be copied from the .slater file with no changes.

The output contains the states section and the density matrices.  One thing to keep in mind is that the density matrices are always labeled from 0 to nmo, where nmo is the number of orbitals in the STATES section.  The original rating of element i,j is STATES[i],STATES[j], where C/C++ ordering is used.

\section example Example

<pre>
method { VMC 
  average { 
       TBDM_BASIS
       mode OBDM
       states { 3 4 5 6 } 
       orbitals {
           CUTOFF_MO
           MAGNIFY 1
           NMO 8
           ORBFILE qwalk.orb
           INCLUDE qwalk.basis
           CENTERS { USEATOMS } 
       }
   }
}
include qwalk.sys
trialfunc { include qwalk.slater } 
      
</pre>

\section options Options

\subsection reqopt Required 

<table>
<tr><th>Option</th><th>Type</th><th>Description</th></tr>
<tr> 
  <td> ORBITALS/CORBITALS </td>
  <td> Section </td>
  <td>
Input for a \subpage MO_matrixDoc. When ORBITALS section is used,
a real-valued wave function is constructed.  When a CORBITALS section is used,
the orbitals will be complex-valued.
</td>
</tr>
</table>


\subsection optopt Optional

While these options are not strictly needed, there are many cases in which the user may want to change them.

<table>
<tr><th>Option</th><th>Type</th><th>Default</th><th>Description</th></tr>


<tr><td>STATES</td><td>Section</td>
<td>All the orbitals listed in the section ORBITALS</td>
<td>A list of the orbitals on which you'd like to evaluate the density matrices.</td>
</tr>

<tr><td>MODE</td><td>String</td><td>TBDM</td>
  <td>TBDM: Evaluate the full two-body reduced density matrix <br>
    OBDM: Evaluate only the one-body reduced density matrix.  This is substantially cheaper <br>
    TBDM_DIAGONAL: Evaluate only the diagonal of the two-body reduced density matrix.  This is somewhat cheaper and uses les memory than the full TBDM, at the cost of giving less information. <br>
</td></tr>


\subsection expopt Experimental options
One shouldn't need to change these options.
<table>
<tr><th>Option</th><th>Type</th><th>Default</th><th>Description</th></tr>

<tr><td>NPOINTS</td><td>Integer</td>
<td>4</td>
<td>Number of auxilliary integration points to use.</td>

<tr><td>NSTEPS_SAMPLE</td><td>Integer</td>
<td>10</td>
<td>Number of Monte Carlo steps to use to randomize the auxilliary integration points.</td>


</table>

*/
